Distribution of lower limb muscles may be associated with the progression of knee osteoarthritis and sarcopenia: a cross-sectional study

Objective This study aimed to delineate and segment the distribution of lower limb muscles based on full-length anteroposterior digital radiography (DR) of patients with varus knee osteoarthritis (vKOA), to quantitatively assess regional muscle abundance, and to explore its associations with vKOA progression and the risk of sarcopenia (SP). Methods A total of 79 vKOA patients (124 lower limbs) admitted to the Joint Center of the Third Affiliated Hospital of Guangzhou University of Chinese Medicine between January 2023 and January 2024 were enrolled according to predefined inclusion and exclusion criteria. Inclusion criteria were: knee pain within the past month, age ≥ 50 years, morning stiffness 30 min, and radiographic findings of joint space narrowing, subchondral sclerosis and/or cystic changes, and osteophyte formation. Exclusion criteria were: valgus KOA or other forms of arthritis, concomitant diseases affecting lower limb function, lower limb deformities of other etiologies, and inability to comply with data collection. Demographic and clinical information, including sex, age, height, weight, body mass index (BMI), diabetes, and hypertension, were collected. From DR images, muscle area indices were extracted from five anatomical sites: femoral lateral muscle, femoral medial muscle, tibial medial muscle, crural interosseous muscle, and lateral fibular muscle. Varus KOA related parameters, including hip–knee–ankle angle (HKA), joint line convergence angle (JLCA), medial joint space width (MJSW), and lateral joint space width (LJSW), were recorded. Appendicular skeletal muscle mass index (ASMI) was obtained using dual-energy X-ray absorptiometry (DXA). Patients were categorized into SP with vKOA (SP-vKOA) and vKOA groups according to the Asian Working Group for Sarcopenia criteria, and into mild vKOA (Kellgren–Lawrence grade 1–2) and severe vKOA (grade 3–4) groups. Intergroup analyses were performed using the rank-sum test, independent-sample t -test, and chi-square test. Binary logistic regression was applied to assess associations between muscle area indices and vKOA severity or SP risk, while linear regression was used to evaluate correlations between muscle distribution ratios and varus deformity. Receiver operating characteristic (ROC) curves were constructed to evaluate diagnostic performance. Results Binary logistic regression indicated that the tibial medial muscle area index (TMMAI) odds ratio ( OR ) = 0.054, 95% confidence interval ( CI ): 0.010–0.302 and weighted average density value (WADV) ( OR = 0.996, 95% CI : 0.993–0.998) were significantly associated with SP occurrence. The combined ROC curve yielded an area under the curve ( AUC ) of 0.852 (95% CI : 0.727–0.881, P 0.001), with a maximum Youden index of 0.594, optimal cutoff of 0.411, sensitivity of 79.17%, and specificity of 80.26%. Age ( OR = 1.081, 95% CI : 1.014–1.152, P 0.05) and the femoral lateral muscle area index (FLMAI) ( OR = 0.120, 95% CI : 0.021–0.697, P 0.05) were significantly associated with vKOA progression. The combined ROC curve showed an AUC of 0.789 (95% CI : 0.709–0.869, P 0.001), maximum Youden index of 0.479, optimal cutoff of 0.533, sensitivity of 76.47%, and specificity of 71.43%. The ratio of femoral medial muscle area index to femoral lateral muscle area index (FMMAI/FLMAI) correlated with varus deformity, being significantly negatively associated with HKA ( t = –2.64, P 0.05), significantly positively associated with JLCA ( t = 4.477, P 0.05), and significantly negatively associated with MJSW ( t = –4.278, P 0.05). Conclusion In patients with vKOA, atrophy of the femoral lateral muscle and imbalance in femoral medial-to-lateral muscle distribution may contribute to vKOA progression, while atrophy of the tibial medial muscle may be linked to the development of SP. Targeted rehabilitation strategies focusing on specific muscle groups in vKOA patients may help alleviate disease progression and reduce the incidence of SP.